1. FIELD OF THE INVENTION
The present invention relates to an automatic transmission mechanism which comprises two planetary gears, and is used with torque converter, in particular, relates to a structure of automatic transmission which functions as a one-way clutch in shifting up and down between third and fourth speed.
2. DESCRIPTION OF THE PRIOR ART
An automatic transmission, in general, comprises a gear unit having two single planetary gear, which is so called Simpson type.
In the above automatic transmission (Simpson type). sun gears are provided in common for both gears, and a carrier of the first planetary gear and a ring gear of the second planetary gear are linked. The driving torque is input to the ring gear and/or the sun gear of the first planetary gear selectively, and certain rotating elements in the planetary gears are restrained by brakes and one way clutches; an output is taken from the ring gear of the second planetary gear which is linked with the carrier of the first planetary gear, then the forward three speeds are obtained.
On the other hand, a four speed automatic transmission comprises three speed automatic transmission mechanism which has the above gear unit having two planetary gears and additional over-drive mechanism or under-drive mechanism comprising one planetary gear so that it can obtain forward four speeds.
However as automobiles have been required to comply with F.F. (front engine and front drive) tendency, a high horse power automatic transmission has been required, wherein minimizing in size and improvement of transmission torque capacity are further required. Under such circumstances the above mentioned four speeds automatic transmission having additional over/under drive mechanism can not satisfy above requirements.
Then, the automatic transmission adding two clutches in the conventional mechanism is proposed to obtain forward four speeds as shown in Japanese patent No. 59-183147. A third clutch is placed between sun gears of the first and second planetary gears and the sun gear of first planetary gear and the carrier of second planetary gear are linked through a fourth clutch. The first and second planetary gears are separated by disconnecting the third clutch, driving torque is input to the carrier of second planetary gear by connecting third clutch, and an output(over drive) is taken from the ring gear by restraining the sun gear of the second planetary gear.
But it is difficult for the above modified Simpson type to embody two planetary gears because they are separated by the clutches and it is difficult to install a few clutches in one place. Accordingly to modify a three speed automatic transmission to that of four speeds requires large modification, so that automatic transmissions have to be manufactured respectively; parts, components, and assembly lines are not designed in common. Due to the above reasons, in the case of small lots and many types, very large cost increase has to be beared, and this can not comply with the recent requirements of variety of car types and their wide variations.